1. Field of the Invention
The present invention relates to data communication systems, and more particularly to local area network data communication systems based on packet communications.
2. Description of the Prior Art
The concept of local area networks is well known (IEEE Project 802, Local Area Network Standards, Draft IEEE 802.3 CSMA/CD Access Method and Physical Layer Specifications, Revision D, IEEE, New York, N.Y., December 1982) and U.S. Pat. No. 4,063,220 issued Dec. 13, 1977, and U.S. Pat. No. 4,099,024.
The traditional configuration for a computer network has been terminals which are controlled by a master computer which receives transmission requests from the respective terminals and grants access to a transmission channel by the individual terminals when the channel is available, usually according to some priority arrangement. The use of such master computers add to the cost of the terminal network and are not required for some networks in which the terminals need communicate only between themselves or with a common storage file. Thus, it is desirable to have a terminal network where the transmission control is imbedded in or shared by the stations making up that network.
Another type of network not having a master controller have employed "contention" schemes or protocol whereby each node of the network contends for the transmission medium whenever it is ready to transmit.
One of the advantages of contention protocols is that they efficiently utilize the available bandwidth on the transmission medium for computer terminal communications. Generally, contention systems permit a user to gain access to the entire channel bandwidth for his message burst. In operation, a user having a message to send is allowed to transmit his message whenever he is ready. In the event another user is presently transmitting over the channel, or does so during the first user's transmission, then a message collision occurs. The system users must sense these collisions and, in response, retransmit their respective messages. Some systems provide random delays before retransmissions to avoid "butting", i.e. continual collisions between users.
The contention protocols are particularly advantageous for computer terminal applications because they take advantage of the low duty cycle or "bursty" nature of data typically being transmitted from terminals and computers. For a large subscriber population of bursty users, the law of large numbers ensures that the channel bandwidth is only required to match the average aggregate data transmission rate of the entire population, rather than matching the sum of the peak rates for bursty subscribers as in the noncontention systems.
One of the earliest contention networks was the "Aloha" system of the University of Hawaii. In this system, each node transmitted whenever it had a packet ready for transmission. Whenever a portion of one node's transmission overlapped with another node's transmission, that overlap destroyed both packets. If the sending node did not receive an acknowledgment within another packet from the destination node after an arbitrary time period, it would assume that a collision had occurred and retransmit. In order to avoid continuously repeated collisions, some method of introducing a random transmission delay had to be introduced.
An attempt at reducing the effects of collision in contention-type networks is disclosed in the Metcalfe et al U.S. Pat. No. 4,063,220 which is directed toward a communication network in which each node is capable of detecting when collision occurs on the channel during that node's transmission, and interrupts its transmission when such a collision occurs. A random number generator is employed to select an interval of time to delay before the next attempted transmission. However, the collision detection mechanism adds to the complexity of the respective nodes with an increase in the cost of the system.
In order to lower the cost of implementation of local area network controllers, it is important to place as many components of the network controller on a single integrated circuit. Since the network controller interfaces between a computer operating at a fixed synchronous clock speed and the cable transceiver operating at a second clock speed the problem of timing and synchronization has presented itself. Prior to the present invention the usual approach toward the design of a local area network controller was to synchronize the computers interface signals with the controllers clock. The controller clock being derived from the cable transceiver data rate.